1. Field of the Invention
The present invention relates to a low-friction ceramic which comprises a matrix phase of a nonoxide ceramic containing silicon.
2. Description of the Prior Art
A method of decreasing the coefficient of friction of ceramics by dispersing silicon carbide (SIC) and boron nitride (BN) in silicon nitride (Si.sub.3 N.sub.4) is disclosed, for example, in Japanese Patent Laid-Open No. 30769/1984. Si.sub.3 N.sub.4 containing an iron oxide such as Fe.sub.3 O.sub.4 as the sintering aid is disclosed, for example, in Japanese Patent Laid-Open Nos. 64268/1983, 88374/1984 [GB No. 8228174 (application date: Oct. 1, 1982)] and 72685/1986 [U.S Ser. No. 631269 (application date: Jul. 16, 1984)].
The Japanese Patent Laid-Open No. 30769/1984 discloses a silicon nitride sinter having a low coefficient of friction. For the purpose of improving the slipperiness at elevated temperatures of said sinter, a filler is dispersed as a solid lubricant in the matrix of the sinter, said filler containing at least one member selected from among carbon, BN and SiC containing free carbon.
The Japanese Patent Laid-Open No. 64268/1983 discloses a silicon nitride sinter and a process for producing the same. Said sinter is prepared by sintering in a nonoxidative atmosphere a powdery mixture comprising 1 to 20% by weight of the powder of at least one oxide selected from among Fe.sub.2 O.sub.3, CaO, TiO.sub.2, MgO, Al.sub.2 O.sub.3, BeO, NiO, Cr.sub.2 O.sub.3, ZrO.sub.2, Y.sub.2 O.sub.3, CoO.sub.2 and SiO.sub.2, 5 to 40% by weight of the powder of at least one silicide selected from among those of metals including Ca, Ti, Zr, Nb, Ta, Cr, Mo, W, Fe, Co, Ni and Y, and the balance comprising Si.sub.3 N.sub.4 powder.
The Japanese Laid-Open No. 88374/1984 discloses a process for producing a silicon nitride ceramic. Said ceramic is prepared by sintering the compact comprising Si.sub.3 N.sub.4 and at least one oxide sintering aid selected from the group consisting of the oxides of metals including Mg, Y, Cr, Mo, Fe, Mn, W, Co, V, U, Ni, Ti, Hf, Zr, Nb and Ta.
The Japanese Patent Laid-Open No. 72685/1986 discloses a corrosion-resistant silicon nitride body. Said body comprises about 3 to 18% by weight of lanthanum oxide, about 0.5 to 5% by weight of aluminum oxide, about 2% by weight of Fe, about 0.2 to 5% by weight of C, Ca, Al or an oxide thereof and the balance comprising Si.sub.3 N.sub.4, and has a relatively high corrosion-resistance against phosphoric acid.
A silicon nitride ceramic is required to have a low coefficient of friction when used as an engine part. In general, Si.sub.3 N.sub.4 containing SiC, BN or an oxide each dispersed therein is poor in the reactivity in the bonding interface and is deteriorated in its own strength, thus causing unfavorable results. The Si.sub.3 N.sub.4 containing an iron oxide such as Fe.sub.3 O.sub.4, as the sintering aid as disclosed in each of the above patent documents is poor in the amount of oxide and cannot attain the characteristic of low friction.